It's unclear how the hormone might increase heart risks.

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Testosterone treatments could boost heart attack and stroke risks, a new study finds.

Testosterone treatments may increase risks for heart attacks, strokes and death in older men with low hormone levels and other health problems, a big Veterans Affairs study suggests.

The results raise concerns about the widely used testosterone gels, patches or injections that are heavily marketed for low sex drive, fatigue and purported anti-aging benefits, the authors and other doctors said.

Men who used testosterone were 30 percent more likely to have a heart attack or stroke or to die during a three-year period than men with low hormone levels who didn't take the supplements. Hormone users and nonusers were in their early 60s on average, and most had other health problems including high blood pressure, unhealthy cholesterol and diabetes.

The research doesn't prove that testosterone caused the heart attacks, strokes or death, but echoes a previous study in older men and should prompt doctors and patients to discuss potential risks and benefits of using the products, said study lead author Dr. Michael Ho, a cardiologist with the VA's Eastern Colorado Health System in Denver.

The nationwide study involved an analysis of health data on 8,700 veterans with low levels of testosterone, the main male sex hormone. All had undergone a heart imaging test and many had risk factors for heart problems, including blocked heart arteries. Risks linked with testosterone were similar in men with and without existing heart problems.

Nearly 26 percent of men using testosterone had one of the bad outcomes within three years of the heart test, compared with 20 percent of nonusers. It's unclear how the hormone might increase heart risks but possibilities include evidence that testosterone might make blood substances called platelets stick together, which could lead to blood clots, the study authors said.

Previous studies on the supplements' health effects have had mixed results, with some research suggesting potential heart benefits but none of the studies has been conclusive.

The new study was published Tuesday in the Journal of the American Medical Association.

An editorial in the journal said it is uncertain if the study results apply to other groups of men, including younger men using the hormone for supposed anti-aging benefits.

"There is only anecdotal evidence that testosterone is safe for these men," said editorial author Dr. Anne Cappola, a hormone expert at University of Pennsylvania and an associate journal editor.

"In light of the high volume of prescriptions and aggressive marketing by testosterone manufacturers, prescribers and patients should be wary" and more research is needed, she wrote.

Annual prescriptions for testosterone supplements have increased more than five-fold in recent years, climbing to more than 5 million and $1.6 billion in U.S. sales in 2011, the study noted.

Dr. Nathaniel Polnaszek, a urologist with Scott & White Healthcare in Round Rock, Texas, said he prescribes testosterone for many men, mostly in their 40s and 50s, with low levels and erectile dysfunction or other symptoms. He called the study "concerning."

"This is something I'm going to be discussing with my patients," he said.

Testosterone levels gradually decline as men age, and guidelines from doctors who specialize in hormone-related problems say hormone supplements should only be considered in men with symptoms of low levels, including sexual dysfunction. They're not advised for men with prostate cancer because of concerns they could make the disease spread, said Dr. Robert Carey, a former Endocrine Society president and a professor of medicine at the University of Virginia Health System.

AbbVie, Inc., makers of one heavily marketed testosterone supplement, AndroGel, issued a statement in response to the study, noting that testosterone treatments are approved by the Food and Drug Administration, and the risks are listed.

Possible side effects shown on the package insert include high blood pressure, blood clots in the legs, and body swelling that may occur with or without heart failure. "This may cause serious problems for people who have heart, kidney or liver disease," the insert says.

"We encourage discussion between physicians and patients that leads to proper diagnosis based on symptoms, lab tests and a patient's other health needs," AbbVie said.